Question

while(!anotherThread.isDone());

or

while(!anotherThread.isDone())
    Thread.sleep(5);

Answer 1

As others have said, it’s better to just use join in this case. However, I’d like to generalize your question and ask the following:

In general when a thread is waiting for an event that depends on another thread to occur is it better to:

1. Use a blocking mechanism (i.e. join, conditional variable, etc.) or
2. Busy spin without sleep or
3. Busy spin with sleep?

Now let’s see what are the implications for each case:

1. In this case, using a blocking call will effectively take your thread off the CPU and not schedule it again until the expected event occurs. Good for resource utilization (the thread would waste CPU cycles otherwise), but not very efficient if the event may occur very frequently and at small intervals (i.e. a context switch is much more time-consuming than the time it takes for the event to occur). Generally good when the event will occur eventually, but you don’t know how soon.
2. In case two, you are busy spinning, meaning that you are actively using the CPU without performing useful work. This is the opposite of case 1: it is useful when the event is expected to occur very very soon, but otherwise may occupy the CPU unnecessarily.
3. This case is a sort of trade-off. You are busy spinning, but at the same time allowing other threads to run by giving up the CPU. This is generally employed when you don’t want to saturate the CPU, but the event is expected to occur soon and you want to be sure that you will still be there in almost real time to catch it when it occurs.